[Gasoline Fumes]

Could be air bubbles in the coolant. That can fool the fast idle system. If the TBI is sitting a lot higher than the radiator, you could try burping the air out at the sensor. Or whatever seems to be the high point of the system. You might want to do this with the radiator cap on. But release any pressure first. Just loosen the sensor a little with the engine running. And of course do everything on a cold engine!

Or try to burb the air out from the cap with the front of the car up on ramps. Or parked on a steep hill.